In nuclear power plants widely spread over the globe, radioactive materials are produced by nuclear fission in nuclear reactors. Examples of the radioactive materials produced as byproducts include radioactive iodine, radioactive strontium, radioactive cesium, radioactive antimony, radioactive ruthenium, and the like, and the effects of these radioactive materials on the human body are concerned. A portion of the radioactive materials produced is also included in vapor and cooling water in a storage vessel in a nuclear reactor. Typically, radioactive materials produced are treated in a nuclear power plant, but for an unexpected reason, such as an accident during treatment of nuclear fuel or a reckless accident in a nuclear reactor, and also in a case such as the Fukushima first nuclear power plant accident which occurred on Mar. 11, 2011, there is a concern in that radioactive materials produced may be released.
Among them, radioactive iodine and radioactive cesium have a low vaporization temperature of 184° C. and 680° C., respectively, and thus are easily vaporized compared to other radioactive materials, so that the radioactive iodine and the radioactive cesium are positioned as three main nuclides in radioactive contamination. As the radioactive iodine, iodine 129 and iodine 131 are main components. Iodine 129 is characterized in that the half-life of iodine 129 is 107 years, which is very long, but the amount of iodine 129 released is small, and the energy of iodine 129 is also low. Meanwhile, iodine 131 is characterized in that the half-life of iodine 131 is 8 days, which is short, but the amount of iodine 131 released is large, and the energy of iodine 131 is high.
Iodine is a trace element which is required to synthesize thyroid hormones in the body and is vital for the human body. When ingested and absorbed in the human body, iodine is collected and accumulated in the thyroid gland in the blood. For this reason, when radioactive iodine is ingested and absorbed, there is a risk that the radioactive iodine is collected in the thyroid gland as in the typical iodine, and as a result, may cause internal exposure of radiation. Since iodine in water may also be in the form of iodic acid which is oxo acid in many cases, iodine and iodine oxo acid need to be treated in order to remove radioactive iodine in water.
As a method for treating radioactive iodine in water, an electrolytic coagulating sedimentation treatment with the addition of silver zeolite has been studied so far (see Patent Document 1). Further, there has been proposed a treatment which includes adding a reducing agent to water to reduce an iodate ion (IO3−), which is difficult to precipitate in the coagulating sedimentation method of the related art, to an iodine ion (I−), and adding silver nitrate to the iodine ion (I−) to produces and precipitates silver iodide (AgI) (see Patent Document 2).
However, the treatment method using coagulating sedimentation has a tendency to increase the running costs because the industrial waste treatment of sludge generated during the coagulating sedimentation treatment also needs to be considered. In addition, an instrument constituting a treatment device extends to various fields such as various chemical injection devices, a precipitation tank, and solid-liquid separation, and also needs a facility with a large installation space.
Furthermore, it is disclosed that iodine adsorbent powder (Ag-13X produced by reacting Zeolite 13X powder with a silver solution, then washing the reaction product with distilled water, drying the product at a suitable temperature of 100° C. or more, and subjecting the product to silver ion exchange in Zeolite 13X powder as an iodine adsorbent powder) cannot adsorb antimony, and as a result, antinomy remains (Patent Document 3).